This invention relates to sealing elements and particularly to a cylindrical sealing element having an annular sealing lip formed of resilient material, bonded to a reinforced base portion also formed partially of resilient material, and a method for its production.
An example of a sealing element in accordance with the invention earmarked for dynamic applications is a piston rubber or packing, as used, for instance, in oil or water-based fluid pumps. For instance, piston rubbers are used in reciprocating, usually double acting, slurry-type pumps in the oil industry for pumping mud during drilling operations. In this application, two piston rubbers are used in a cylindrical liner in assembly with the normal piston hub and retainer and locking rings. A piston rubber has a generally cylindrically shaped elastomeric body with a lip seal bonded to a reinforced base or heel. Pairs of piston rubbers are arranged back-to-back with their reinforced ends abutting the piston hub with the lip seals facing away from one another and sealingly engaging the liner bore.
An example of a sealing element used in static applications is the stopple sealing element used to temporarily seal off a pipeline for plugging or bypass operations. The section of line is isolated by lowering the stopple sealing element through a special fitting and then rotating the sealing element, which is attached to a steel plate, so that a seal is attained and flow in the pipeline may be temporarily stopped. The stopple sealing element has a sealing lip similar to the piston rubber, and is attached to a reinforced cylindrical base portion to which the steel plate is typically attached with fastening members.
The standard reinforced piston rubber and stopple sealing element base portion are formed of a multi-layered fabric reinforcement made by stacking a number, e.g., 5-30, of annular rings (in the case of the piston rubber) or disks (in the case of the stopple) which are individually died out in "cookie-cutter" fashion from flat, square-woven fabric impregnated with elastomer. Cut threads of the square-woven fabric are exposed along a circumferential portion of the heel or base portion of the sealing element to define a wear-resistant surface. Another type of piston rubber reinforcement is fabricated by spiralling a flat strip of square-woven fabric to cause the threads of the fabric to slip and change angles relative to each other as the flat strip of fabric is helically applied to form an annulus. This piston rubber construction and method are disclosed in U.S. Pat. No. 3,719,366 to Pippert. Like the cookie cutter method, cut ends of the helically spiraled fabric are exposed at a circumferential portion of the piston rubber.
Another type of piston rubber construction is disclosed in U.S. Pat. No. 4,280,709 to Heikes et al. In that patent, the reinforcement is made by wrapping a square-woven textile fabric in volute fashion to form a roll. The roll is square cut to form a plurality of bands or rings of desired width. The band is positioned during manufacture of the piston rubber so that threads of the fabric have side portions exposed along a circumferential end portion of the piston rubber to define a wear-resistant surface. The method of manufacture substantially reduces fabric scrap (in contrast to the waste produced by the cookie cutter method), simplifies fabrication and also yields a piston rubber construction having improved wear-resistance. Despite these advantages there has been a tendency for the helically rolled fabric to come unraveled in use because or poor bonding at the distal seam.
For sealing elements of the aforementioned type, failures have largely occurred in the base or heel area of the molded elastomeric elements. Attempts have been made to overcome these problems by utilizing different fabric orientations (as in the Heikes et al. patent), or use of higher modulus materials for the base section such as steel or polyurethane, for instance. Some sealing elements have even been made exclusively of elastomer without any reinforcing members. While some of these alternate constructions have proved comparable in performance to the standard plied up cookie cutter construction, none appear to offer significantly longer service life which has been a goal in the industry.
A thorough examination of piston rubber wear and failure was presented by J. R. Arnett in "Slush Pump Expendables, Performance and Economics," 1981 Drilling Conference Transactions. International Association of Drilling Contractors, 1981, pp. 269-277. Arnett found in both theoretical examination and failure analysis that the predominant factors in noncatastrophic piston rubber failures was abrasion and, to a lesser extent, erosion and adhesion. Catastrophic failures of properly manufactured piston rubbers were found to be mainly due to delamination in the heel section and at the transition between heel and seal areas. The paper found that no present combination of materials and design economically improved the life over the conventional design.
It is an object of this invention to provide a sealing element having a reduced tendency of the sealing lip and reinforced base portions to delaminate.
It is another object to produce a sealing element with an integrated structure having a greater elastomer-to-elastomer chemical bonding surface and a reduced volume and density of the reinforcement material within the sealing element compared to prior reinforced constructions.
It is another object to provide a simplified method of construction of the sealing element characterized by reduced scrap produced during manufacture.
It is still another object to provide a sealing element of improved performance and life through enhanced wear and stress fatigue resistance.